Look at the things they didn't say about the pope last night. Cameron and Brown are both privately devout Christians. Neither mentioned this in the debate; it won't win votes. Clegg is an atheist and said so, presumably because there are some votes in that; but he said it in a very moderate way ("I'm not a man of faith") and balanced it immediately with the information that his wife and children are Catholics.

They all said that they welcomed the pope's visit; that they were opposed to child abuse and all either said or suggested that they disagreed with his teachings on sexuality. It would have been quite a story had any of them said anything different. No party leaders suggested they might learn anything from the churches. The obvious question is why not. There are at least twice as many regular Mass goers as there are members of all the political parties put together. Why don't they ask what the pope has got that they have not?

It's a commonplace of debate on here that Christianity is shrinking and ageing and deeply unrepresentative. But the Christian churches are not shrinking, ageing nor nearly as shrunken as any of the political parties. The best figures I can find on these come from a paper in the House of Commons library: they are a little out of date, because the political parties don't publish their members as often or as clearly as the mainstream churches do. The Conservatives, in 2008, had about a quarter of a million members, down from 3m in the Fifties; the Labour party had 166,000 members

The Liberal Democrats, who maintain that bishops in the Lords are an undemocratic anachronism, have 60,000 members.

There is a real problem for politicians here, and perhaps for democracy: even though most citizens have no strong ideological commitments of any sort, when they do they care more about their religious commitments than their political ones. I think this is true all across Western Europe now. It didn't use to be. When I moved to Sweden in 1977, politics was still, just, a mass activity. Party, or union, membership reached out into almost everyone's lives and was supposed to supply a moral dimension. That's not true now, although Swedish politicians of every sort still feel themselves part of a single moral community with all other Swedes in a way that the English just don't.

The decline of social commitment is general in modern consumerist societies, and affects everything from stamp collecting to organised sport. It's not just politicians and religious leaders who must contend with it. But I think religious leaders are more alive to the existential threat it poses, and in some ways are responding better.

None of this – to short-circuit a tedious and predictable debate – is an argument that any particular religious belief is true; nor that any particular political belief should be. But societies can't work purely on compulsion. They need large-scale, co-ordinated voluntary action. That doesn't have to be religious: indeed the social democratic movements of the 20th century were explicitly irreligious as well as secular, but they're all over now. And you can't have large-scale co-ordinated voluntary action without faith: a combination of self-discipline and hope in an uncertain future state.

This definition of "faith" has nothing to do with propositional truth, and quite right too. It is much closer to "trust". But it is also mutual. You have it within a group, if not a community, and you lose it when other members of the group show they don't have it. And when you lose it, you stop co-operating; you stop doing anything you're not forced to do.

This country is faced with some really nasty choices in the next few years, as all the debating politicians know, and none dare say. They don't have faith in us, either. Yet religious leaders and their followers still, and despite their scandals, have that mutual trust.

In modern British politics, if you say you have a religious faith you're weird. But the problem for our politicians is that if you say you have a political faith you're just embarrassing and sad. So they sell themselves on personalities and even policies instead. Who's going to believe in that?